At boot time, import the Tails signing key into tails-iuk-get-update-description-file's trusted GnuPG homedir.

It is needed by tails-iuk 0.1.6-1.
libarchive-tar-wrapper-perl 0.16-1 is in NEW.

The upcoming tails-iuk 0.1.6 needs that.

Namely:
The default Live user (`amnesia`) runs the update frontend and:

* is allowed to run a `/usr/local/sbin/network-shutdown` script, using
  passwordless sudo, as root.
* is allowed to run the `tails-install-iuk` program, with any
  arguments, using passwordless sudo, as the `tails-install-iuk` user.

The `tails-install-iuk` user is allowed to run, using passwordless
sudo, every command required by its task (currently: `mount`, `cp`,
`rm` and `tar`).

This is achieved by appending module=Tails to the kernel command-line.

The default is to save them at ~/Mail, which isn't included in our
current Claws Mail persistence preset.

This version, which works for us, will soon be replaced in testing by another
one that does not work for us.

Revert "Install plymouth from testing."
This reverts commit fbb161af.

We've been using plymouth from unstable, but it now depends on
multiarch-support. The proper solution would be backporting, but we
need things to move forward a bit more quickly for the upcoming 0.12.1
release.

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